moved the check to doCopyNode and Fetch

Signed-off-by: wangxiaoxuan273 <wangxiaoxuan119@gmail.com>

Author: wangxiaoxuan273

content.go

@@ -26,6 +26,7 @@ import (
 	"oras.land/oras-go/v2/content"
 	"oras.land/oras-go/v2/errdef"
 	"oras.land/oras-go/v2/internal/cas"
+	"oras.land/oras-go/v2/internal/descriptor"
 	"oras.land/oras-go/v2/internal/docker"
 	"oras.land/oras-go/v2/internal/interfaces"
 	"oras.land/oras-go/v2/internal/platform"
@@ -273,6 +274,9 @@ func Fetch(ctx context.Context, target ReadOnlyTarget, reference string, opts Fe
 		}
 		rc, err := target.Fetch(ctx, desc)
 		if err != nil {
+			if errors.Is(err, errdef.ErrNotFound) && descriptor.IsForeignLayer(desc) {
+				return ocispec.Descriptor{}, nil, fmt.Errorf("the artifact with foreign layer %s is not supported: %w", desc.Digest, err)
+			}
 			return ocispec.Descriptor{}, nil, err
 		}
 		return desc, rc, nil
@@ -291,6 +295,9 @@ func Fetch(ctx context.Context, target ReadOnlyTarget, reference string, opts Fe
 	proxy.StopCaching = true
 	rc, err := proxy.Fetch(ctx, desc)
 	if err != nil {
+		if errors.Is(err, errdef.ErrNotFound) && descriptor.IsForeignLayer(desc) {
+			return ocispec.Descriptor{}, nil, fmt.Errorf("the artifact with foreign layer %s is not supported: %w", desc.Digest, err)
+		}
 		return ocispec.Descriptor{}, nil, err
 	}
 	return desc, rc, nil

copy.go

@@ -26,6 +26,7 @@ import (
 	"oras.land/oras-go/v2/content"
 	"oras.land/oras-go/v2/errdef"
 	"oras.land/oras-go/v2/internal/cas"
+	"oras.land/oras-go/v2/internal/descriptor"
 	"oras.land/oras-go/v2/internal/platform"
 	"oras.land/oras-go/v2/internal/registryutil"
 	"oras.land/oras-go/v2/internal/status"
@@ -258,6 +259,9 @@ func copyGraph(ctx context.Context, src content.ReadOnlyStorage, dst content.Sto
 func doCopyNode(ctx context.Context, src content.ReadOnlyStorage, dst content.Storage, desc ocispec.Descriptor) error {
 	rc, err := src.Fetch(ctx, desc)
 	if err != nil {
+		if errors.Is(err, errdef.ErrNotFound) && descriptor.IsForeignLayer(desc) {
+			return fmt.Errorf("the artifact with foreign layer %s is not supported: %w", desc.Digest, err)
+		}
 		return err
 	}
 	defer rc.Close()

internal/descriptor/descriptor.go

@@ -18,6 +18,7 @@ package descriptor
 import (
 	"github.com/opencontainers/go-digest"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"oras.land/oras-go/v2/internal/docker"
 )
 
 // Descriptor contains the minimun information to describe the disposition of
@@ -45,3 +46,15 @@ func FromOCI(desc ocispec.Descriptor) Descriptor {
 		Size:      desc.Size,
 	}
 }
+
+func IsForeignLayer(desc ocispec.Descriptor) bool {
+	switch desc.MediaType {
+	case ocispec.MediaTypeImageLayerNonDistributable,
+		ocispec.MediaTypeImageLayerNonDistributableGzip,
+		ocispec.MediaTypeImageLayerNonDistributableZstd,
+		docker.MediaTypeForeignLayer:
+		return true
+	default:
+		return false
+	}
+}

registry/remote/repository.go

@@ -35,7 +35,6 @@ import (
 	"oras.land/oras-go/v2/content"
 	"oras.land/oras-go/v2/errdef"
 	"oras.land/oras-go/v2/internal/cas"
-	"oras.land/oras-go/v2/internal/docker"
 	"oras.land/oras-go/v2/internal/httputil"
 	"oras.land/oras-go/v2/internal/ioutil"
 	"oras.land/oras-go/v2/internal/registryutil"
@@ -615,9 +614,6 @@ type blobStore struct {
 
 // Fetch fetches the content identified by the descriptor.
 func (s *blobStore) Fetch(ctx context.Context, target ocispec.Descriptor) (rc io.ReadCloser, err error) {
-	if isForeignLayer(target) {
-		return nil, fmt.Errorf("the artifact with foreign layer %s is %w", target.Digest, errdef.ErrUnsupported)
-	}
 	ref := s.repo.Reference
 	ref.Reference = target.Digest.String()
 	ctx = registryutil.WithScopeHint(ctx, ref, auth.ActionPull)
@@ -1432,14 +1428,3 @@ func generateIndex(manifests []ocispec.Descriptor) (ocispec.Descriptor, []byte,
 	indexDesc := content.NewDescriptorFromBytes(index.MediaType, indexJSON)
 	return indexDesc, indexJSON, nil
 }
-
-func isForeignLayer(desc ocispec.Descriptor) bool {
-	switch desc.MediaType {
-	case ocispec.MediaTypeImageLayerNonDistributable,
-		ocispec.MediaTypeImageLayerNonDistributableGzip,
-		ocispec.MediaTypeImageLayerNonDistributableZstd,
-		docker.MediaTypeForeignLayer:
-		return true
-	}
-	return false
-}